United States stocks edged higher, while the Nasdaq Composite Index breached 4,000 for the first time in 13 years.
The Dow Jones Industrial Average rose 27.68 points, or 0.17%, to 16,092.45 points.
The S&P 500 Index added 1.14 points, or 0.06%, to 1,805.90 points.
The Nasdaq Composite Index rose as high as 4,007 in early trading, though it pared those gains to recently climb 9.02 points, or 0.23%, to 4,000.67 points
A surprise deal over the weekend to curb Iran's nuclear program in exchange for a loosening of economic sanctions boosted sentiment during a holiday trading week, particularly with few other market movers on tap Monday, investors said.
News of the deal sent oil prices falling 1% and lifted European markets.
"In light of the fact that you have fewer people around, modestly positive news such as the Iran deal becomes more positive," said Dan Greenhaus, chief global strategist at BTIG. "All else equal, lower oil prices are better than higher oil prices."
The Nasdaq Composite's push through the 4,000 level has been driven largely by stocks outside the technology sector. Health-care and consumer stocks increasingly comprise the bulk of the index, which at the 4000 level has climbed 33% this year, a full 10-percentage-point lead over the Dow industrials.
Despite the recent gains, though, the Nasdaq remains 21% below its all-time high, even as the Dow repeatedly touches new record highs. The Nasdaq index peaked at 5,048.62 on March 10, 2000.
On Friday, the Dow rose 55 points, or 0.3%, to a sixth record high close in eight sessions. The Dow also posted a seventh-straight weekly gain, the longest such stretch since January 2011.
The S&P 500 closed above 1,800 Friday for the first time to post its 37th record high close of the year.
The Federal Reserve's easy-money policy, low inflation, a slow-but-steady economic expansion and a shrinking number of attractive alternatives have helped fuel the stock market's push into record territory in recent months.
"The path of least resistance is higher," said Wayne Kaufman, chief market analyst at Rockwell Securities. "The economy has been doing pretty well ... We're fine into next quarter, but the market is definitely becoming more selective."
Lower oil prices come at a particularly advantageous time because any decline in gasoline prices means more cash in consumers' wallets going into the holiday shopping season, said Joe Tanious, global market strategist at J.P. Morgan Asset Management.
"That bodes well for consumption and retail sales going into arguably the most important month of the entire year," Mr Tanious said. "Everybody gets into a car and everybody's about to go out there and start spending. With more disposable income in consumers' pockets, it really couldn't come at a better time."
Gasoline prices in the US have fallen steadily for much of this year, weighed down by weak demand amid the sluggish recovery and elevated supplies. Prices at the pump averaged $3.28 for a gallon of regular on Monday, according to auto club AAA, down from $3.43 a year ago.
January crude-oil futures slid 1% to $93.87 a barrel, as the weekend deal will make it easier for Iran to export some of its crude oil to international markets. December gold futures shed 0.3% to $1,241.20 an ounce and hit a 4 1/2-month low earlier in the session, as the deal weighed on assets seen as safe havens.
The yield on the 10-year Treasury note inched lower to 2.737% from 2.754% late Friday.
In Europe, the Stoxx Europe 600 rose 0.4% to trade just below 5 1/2-year highs reached early last week. Also bolstering sentiment, data showed that French business confidence in November was unchanged from October levels, versus expectations of a slight decline.
Germany's DAX 30 index rallied 0.9%, France's CAC tacked on 0.4% and the U.K.'s FTSE 100 rose 0.2%.
The dollar rose, reaching a six-month high against the yen and edging higher against the euro.
Asian markets were mostly higher, highlighted by the 1.5% rally in Japan's Nikkei Stock Average to a six-month high. Chinese stocks bucked the regional trend, with China's Shanghai Composite slipping 0.5% after gaining 2.8% last week.
In corporate news, J.C. Penney shrugged off news late Friday that S&P Dow Jones Indices would strike the retailer's stock from the benchmark S&P 500 Index and move it to the S&P MidCap 400.
The decline in crude-oil prices was benefiting some airliner stocks. Delta Air Lines touched an all-time high early in the session. Southwest Airlines also gained.
